The present invention relates to a machine installation comprising a safety switch for generating a system enable signal, and, more particularly, to a safety switch for generating a system enable signal in response to the position of a movable guard door for safeguarding a hazardous area of a machine.
Safety switches of this kind may also be referred to as guard locking devices having a failsafe guard lock. They are desired on guard doors, guard flaps or the like, i.e. on moveable members intended to temporarily prevent access to an automated machine or tool as long as the machine or tool is in a hazardous operating state. The machine may be, for example, a robot, a machine tool having a rotating spindle, a transportation or conveying system, a press or any other machine whose operation presents a hazard to people located in the working area of the machine. The safety switches are used as signaling devices for signaling the closed position of the guard door to a control unit, so that the control unit is allowed to control automated operation of the machine. Typically, the control unit is designed to allow start-up of the machine only when the guard door is closed. If the guard door is opened during machine operation, the control unit must bring the machine into a non-hazardous state by switching off, for example, the power supply to the machine.
There are some machines which still represent a hazard for a certain amount of time even after they have been shut down, for example because a drive of the machine needs time to come to a final standstill. For such situations, safety switches are desired which prevent the guard door from being opened until the machine has reached its non-hazardous state. This function is sometimes referred to as a guard lock.
Typical safety switches involving a guard lock have a bolt (sometimes called an actuator) which is typically arranged on the movable guard door part. When the guard door is closed, the bolt may engage in a receptacle on the door frame, which is detected with the aid of one or more sensors. In the case of a safety switch with a guard lock, the bolt is also locked in the bolt receptacle, preventing it from being withdrawn. In some cases, the bolt may perform two functions, namely that of a detector element, with the aid of which the closed position of the guard door can be detected, and secondly that of a “true” bolt which prevents the guard door from being opened as long as the bolt is locked in the bolt receptacle. The bolt may be released via an electromotive actuator which is activated by the control unit when the monitored machine or system has reached its safe state.
DE 10 2005 057 108 A1 discloses a safety switch having the functions of a guard lock and a closed door monitor. This safety switch does not have any option for releasing the guard door. In practice, a release signal required to unlock the guard door is generated in a remote safety switching device using a switch located in the region of the safety switching device. The spatial distance is disadvantageous if the safeguarded machine or system is intended to be operated in special operating modes, such as during set-up or fault correction. The spatial distance also makes it difficult to quickly intervene in the machine, for example in the event of a malfunction.